1. Field of the Invention
This invention relates to a power generating apparatus suitable for use as an a.c. generator for an automotive vehicle.
2. Description of the Related Art
In order that a small-sized and light-weight a.c. generator having a small-sized rotor can generate a high power output, its rotor is required to generate a large amount of magnetic flux. A method for attaining the above requirement in such an a.c. generator is known in which a permanent magnet is used in combination with the field winding so a to generate a large amount of magnetic flux. However, this method has the drawback that the field magnetomotive force in the generator cannot be satisfactorily controlled. A method which solves the above problem and attains the desired control of the field magnetomotive force in an a.c. generator is disclosed in, for example, JP-A-63-77362. According to the disclosure of JP-A-63-77362 cited above, the centrifugal force is utilized to cause displacement of part of the magnetic path in such a generator thereby regulating the amount of generated magnetic flux. Also, U.S. Pat. No. 4,882,515 discloses an a.c. generator in which a field coil and a permanent magnet are disposed between a set of claw-type magnetic pole members so as to couple two magnetic circuits thereby regulating the amount of generated magnetic flux.
The former cited above is advantageous in that the output of the permanent magnet relative to the rotation speed of the generator can be regulated to produce a predetermined field magnetomotive force so as to maintain constant the output voltage of the generator. However, the former has a problem that a variation of the load demand leads to a corresponding variation of the output voltage of the generator.
On the other hand, in the case of the latter cited above, the ends of the claw-type magnetic pole members are required to be accurately positioned relative to each other so as to reliably couple the two magnetic circuits. Thus, the later has the problem that the assembling of the components of the generator is difficult, and the structure of the generator becomes inevitably complex.